1. Field of the Invention
This invention relates to a process for purifying a 1,2-unsaturated carboxylic acid (otherwise known as an alpha, beta-unsaturated carboxylic acid or a 2-unsaturated carboxylic acid) and/or its esters.
In accordance with the process of the present invention, aldehydes contained as impurities in 1,2-unsaturated carboxylic acids and/or their esters can be removed efficiently by simple operations.
2. Prior Art
1,2-Unsaturated carboxylic acids such as acrylic acid, methacrylic acid, etc. and/or their esters have a broad range of uses as starting materials for synthesis of high-molecular compounds.
These 1,2-unsaturated carboxylic acids have been produced industrially through a vapor-phase catalytic oxidation reaction of alkenes and/or alkenals, etc., in recent years. However, the 1,2-unsaturated carboxylic acids such as acrylic acid and methacrylic acid produced according to the above method contain various types of impurities such as aldehydes, e.g., formaldehyde, acetaldehyde, propionaldehyde, isobutylaldehyde, acrolein, methacrolein, furfural, and benzaldehyde; ketones, e.g., acetone; or acids, e.g., formic acid, acetic acid, and propionic acid.
A considerable amount of these impurities are separated and removed by repeating conventional purification methods such as distillation, extraction and the like.
However, aldehyde impurities remain in the 1,2-unsaturated carboxylic acids, because (i) aldehydes are newly formed during the above mentioned purification operations such as distillation and extraction (for example, acrolein contained in acrylic acid is thermally converted to acrolein dimer during the purification process of the acrylic acid) and (ii) aldehydes such as furfural and benzaldehyde have boiling points close to that of acrylic acid and separation thereof from the acrylic acid by distillation operation is difficult.
On the other hand, in the industrial production of 1,2-unsaturated carboxylic acid esters, these esters are usually obtained by esterification of the corresponding 1,2-unsaturated carboxylic acids which have been produced at a low cost according to the above-mentioned industrial method and purification by simple distillation. Thus, the resulting 1,2-unsaturated carboxylic acid esters are inevitably contaminated with aldehyde impurities as mentioned above.
When such 1,2-unsaturated carboxylic acids and/or their esters containing the aldehyde impurities are used as starting materials for synthesis of high-molecular compounds, these impurities cause various problems such as a polymerization-inhibition effect and coloring of the resulting polymers.
Examples of methods for removing aldehyde impurities contained in 1,2-unsaturated carboxylic acids and/or their esters are a distillation method; an adsorption method using activated carbon or molecular sieves having an amine compound adsorbed thereon (Japanese Laid-Open Patent Specification No. 18934/81); and a method of forming sulfite-addition products by the addition reaction of sodium bisulfite; a method comprising addition of an amine compound (Japanese Patent Publication No. 31087/73) or addition of an amino acid such as glycine (Japanese Patent Publication No. 14/80) have been known.
However, in the distillation method, acrolein exhibits complicated behavior during purification. For example, during purification of acrylic acid, for example, the acrolein is readily dimerized into acrolein dimer during the distillation process as mentioned above or reacts with water to form .beta.-hydroxylpropionaldehyde. On the other hand, large numbers of distillation plates and a large reflux ratio are required for the sufficient separation of aldehydes such as furfural and benzaldehyde because the difference in boiling point between these aldehydes and acrylic acid is small and because these aldehydes have small relative volatility against acrylic acid. It is disadvantageous from the industrial point of view to conduct rectification of polymerizable acrylic acid at a high temperature and under such severe conditions because special means and the like are required for preventing polymerization of the acid.
On the other hand, the adsorption method by means of activated carbon or the above mentioned molecular sieves requires a troublesome regeneration operation of the adsorbent used and, moreover, is unsatisfactory for the removal of trace amounts of impurities. The method comprising formation of sulfite addition products by the addition reaction of sodium bisulfite has almost no effectiveness in the case of purification of, for example, a 1,2-unsaturated carboxylic acid.
The method comprising addition of an amine compound or an amino acid fails to satisfactorily remove aldehyde impurities. Moreover, it is generally known that amine compounds have a polymerization-promoting effect on 1,2-unsaturated carboxylic acids and their esters. Also, some amines such as anhydrous hydrazine are unstable and dangerous, which gives rise to problems of process operation and safety.